A depth-resolving numerical model of physically forced microbiology at the European shelf edge

SEDBIOL, a 1-D depth-resolving model, which couples physical, microbiological, and sedimentation/resuspension submodels, is described. The physical submodel uses a level 2 turbulence closure scheme forced by meteorology and tidal elevations to provide seasonally varying turbulent diffusivites. Within this framework is placed a model for water column microbiology, which includes the cycling of nitrogen and carbon through microplankton (phytoplankton and pelagic microheterotrophs) and detrital compartments. The microbiological system is closed by mesozooplankton grazing pressures taken from observed zooplankton abundance. Inorganic and organic (microplankton and detritus) light-attenuating particulates sink through the water column depositing at the seabed to form a superficial layer of ‘fluff’ from which they can be resuspended. The model has been used to simulate conditions on the Goban Spur (49.5°N 10.5°W) in 200 m of water, using climatological meteorological forcing, M2 tidal dynamics and zooplankton data from the Continuous Plankton Recorder (CPR) Survey. The results were compared with observed temperature, chlorophyll, nitrate and annual primary production data taken as part of the CEC MAST II Ocean Margin EXchange (OMEX) project, and with remotely sensed chlorophyll. The physical submodel was calibrated against observed temperature profiles, and its fit improved by adding ‘pseudo-weather’ and a relaxation term for deep-water temperature, and by increasing the value of ‘background’ eddy coefficients. The sensitivity of the microbiological model to changes in the physical environment and several key parameters has been tested, with values improved by fitting simulations to observed chlorophyll and nitrate concentrations. The coupled model was used to predict annual net primary productivity, carbon fluxes into near-bed and fluff layers and to compare the magnitude of biological and physical terms.